In prior art, a sub-chassis of a crane moving on rubber-tired wheels comprises two large wheels one after the other, and the wheels can be turned to be parallel for lateral movement. The turning is conducted by turning the wheels to the same direction with a hydraulic cylinder or aggregate in a place specifically reserved for turning, e.g. on a marble slab. Another possibility is to lift the wheels from the ground for the duration of the turning action. Instead of two large wheels, the sub-chassis may comprise a pair of small wheels, both of which are drive wheels. Power transmission is usually implemented by open gearing with chain gears and transmission chains.
U.S. Pat. No. 3,081,883 discloses a solution in which a sub-chassis of a crane comprises four parallel wheels, the two on the outer edges being drive wheels. The sub-chassis can be turned by operating the chain-geared traversing gears of the drive wheels to different directions.
A disadvantage of the prior art arrangement is that turning is difficult. There are only two operating positions, 0.degree. and 90.degree., and it is extremely difficult, practically impossible, to steer the crane diagonally e.g. for servicing purposes. Also, because of hydraulic equipment, many components are needed, and so there are many points that may leak. Further, in chain gearing there are many points that may need servicing, and in addition chain gearing requires space.